1. Field of the Invention
This invention relates to a shutter control device in an automatic exposure camera of the shutter speed control type and which has a device for effecting so-called shutter release lock, i.e., wherein shutter release cannot take place even though shutter release operation is effected during consumption of a battery.
2. Description of the Prior Art
The prior art will hereinafter be described by taking as an example a single lens automatic exposure camera of the shutter speed control type having a focal plane shutter. Such a camera usually includes a rearward shutter curtain restraining electromagnet. In order to prevent unnecessary consumption of the battery, this magnet is not at all times energized, but is energized simultaneously with the mirror-up operation. That is, the rearward curtain, which is mechanically restrained before movement of the forward shutter curtain, becomes electromagnetically restrained by this magnet when the magnet is energized simultaneously with the mirror-up operation effected by depression of a shutter button. When the time from the start of movement of the forward curtain has become equal to the exposure time calculated by an exposure operating circuit, the power supply to the magnet is cut off and the restraint of the rearward curtain is released to permit it to move. In this manner, photography with a proper exposure is completed.
When the battery has been consumed and the battery voltage has dropped to a level unsuitable for effecting exposure control, there is generally known a device for detecting the battery voltage and effecting shutter release lock. The shutter release lock assures that even if a shutter release operation such as depression of the shutter button is effected, the shutter is not operated, that is, shutter release does not take place. The shutter release lock is usually accomplished in a manner to be described hereinafter.
In a predetermined time after the upward movement of the mirror, a forward curtain restraining mechanism is released and the forward shutter curtain starts to move. This forward curtain restraining mechanism is released to permit the forward curtain to start moving after the rearward curtain restraining electromagnet has been energized and the restraint of the rearward curtain has been confirmed. Accordingly, during a predetermined time after mirror-up, the forward curtain restraining mechanism may become releasable by the energization of the rearward curtain restraining electromagnet (namely, the power supply to the magnet coil). Therefore, the device is designed such that when the battery voltage has dropped below a reference value, such drop is detected to prevent the power supply to the rearward curtain restraining electromagnet coil after mirror-up, the magnet will not be energized and accordingly, release of the forward curtain restraining mechanism cannot take place. In this manner, shutter release lock during consumption of the battery is accomplished. Such a technique is disclosed, for example, in U.S. Pat. No. 4,096,503. However, in automatic exposure control, there occur cases where a very short shutter speed above the allowed highest shutter speed (for example, 1/1000 sec.) is required. Such a condition may occur is the object brightness is not extremely high but the set aperture value is sufficiently small. At such time, it may occur that the rearward curtain restraining electromagnet is not energized from the beginning and therefore, the condition in which the forward curtain restraining mechanism can be released cannot be brought about. Accordingly, even in such a condition, there is a possibility that the shutter release lock works. When the shutter release lock takes place as a result, the photographer cannot determine whether it is due to the drop of the battery voltage or to the fact that the shutter time is much shorter than the design program and accordingly, he may sometimes encounter difficulty in judging what measure is to be taken thereafter.